1. Field of the Invention
The present invention relates to a method of producing a high precision optical glass element such as a lens, a prism or the like and a preform of the optical glass element used for reheat press forming, and to an apparatus for producing the optical glass element.
2. Description of the Prior Art
In recent years, optical glass lenses have a tendency to be formed into an aspheric shape which permits both the simplification of the lens structure and the minimization in weight of the lens portion. In the production of such aspheric lenses, since they cannot be easily worked and mass-produced by a grinding method, which is a conventional method of producing optical lenses, a forming method using a mold is considered to be promising (refer to U.S. Pat. Nos. 3,900,328, 4,139,677 and 4,168,961). This forming method using a mould is a method in which a polished optical glass is formed by heating on an aspheric mould which is finished to a surface with desired surface quality and surface accuracy or a gob of molten optical glass is formed by heating.
When optical glass elements such as aspheric lenses, prisms and the like are produced by reheat press moulding, it is necessary to use optical glass preforms having surfaces without any pit such as fine irregularity or flaws (for example, in a mirror surface having surface roughness of 0.005 .mu. or less by RMS). Conventional preforms of optical glass are formed by receiving molten glass in a mould or by working molten glass and then polish it (refer to U.S. Pat. No. 4,139,677 and Japanese Patent Unexamined Publication No. 59-116137). The method of working molten glass and then polish it requires many complicated processes and is thus expensive. A method of working high temperature glass by using a glass former such as a shaping pan, a mould or the like, which is coated with a parting agent carbon layer by gas phase reaction, is proposed as the method of receiving molten glass (refer to, for example, Japanese Patent Unexamined Publication No. 54-60312). Since the parting agent carbon layer formed in this method comprises a porous heterogeneous film, there are problems in that the parting agent adheres as a foreign substance to the glass surface, a mirror surface with surface roughness of 0.005 .mu. or less by RMS cannot be obtained, fine irregularity occurs in the glass surface, and the life of the mould used is reduced owing to the oxidative wear of the carbon layer. In a method proposed for removing pits from the glass surface, the thickness as the center of the original glass form having a central thickness which is greater than that of the preform is reduced by softening by heating on a support plate made of a fireproofing material such as graphite, mullite or cordierite (refer to, for example, Japanese Patent Unexamined Publication No. 64-52619). Since the fireproofing support plate used in this method is made of a porous material and has fine irregularity, the fine irregularity is transferred to the contact surface of the preform from the fireproofing support plate, or blow holes are produced. No optical mirror surface can be thus obtained, and the preform is subjected to after-working (such as polishing) for removing fine irregularities after the production of the preform.